1. Field of the Invention
The pin bodies, which serve to split up and rearrange or fold over the mass flow as a result of considerable turbulences in the otherwise predominantly laminar flow directly within the worm-lands or threads of the screw, and hence serve for intensive mixing of the extrusion material, are generally disposed in a plurality of radial planes which are respectively distributed in a star-shaped orientation over the length of the extruder housing. Extruders which are provided with such screw housings, generally known as "pin cylinders" or also "cross-current mixing cylinders", have proven themselves in operation, and have completely fulfilled expectations associated with the generation of profiled strands having improved homogeneity. However, failure or breakage of individual pins surprisingly occurred over and over again during operation; these failures generally also resulted in further secondary or consequential damages. Since at the beginning of the kneading or mixing process, the extrusion mass is still relatively viscous and does not flow readily, primarily those pin bodies which are located in the entry region of the extruder are struck, so that under the effect of the pressure of the plastic mass flow, which is particularly great in this region, particularly those pins located here are subject to failure or breakage. During further transport to the nozzle or orifice of the extruder, the loose metallic pieces of the broken pins are carried along and cause even worse damage to the subsequently arranged pin bodies as well as to the screw, the housing walls, and the nozzle of the extruder. These damaging effects can even be observed outside the extruder on other subsequently arranged equipment, such as calender rolls. Since the pin bodies project with only slight lateral free space into the recesses expressly provided therefor in the lands or threads of the screw, even slight deformations which are short of a break are already enough to bring them into direct contact with the threads of the screw, resulting in either their complete breaking off or at least to deformation or a breaking off of the threads of the screw. In all cases, lengthy and expensive repair work is unavoidable. All attempts to prevent the danger of breakage by means of a special shape of the pin bodies, have proven up to now to be unsuccessful.